"Mind-reading" has become a common topic in popular culture, suggesting that careful observation of a person’s facial features may reveal covert thoughts. The scientific evidence behind the human skill to infer relations between such outwardly visible markers and another individual's cognition is remarkably little and even less is known about whether humans can acquire these from experience. Pupil dilation has been related to a number of cognitive processes and is thus a potential outwardly accessible marker for betraying one's thoughts. Here we tested, with an adapted version of the childhood game "rock-paper-scissors", whether players can increase their chance of winning by exploiting their opponent’s pupil. Opponents played a series of games against a computer, while we recorded videos of their eyes. The words rock, paper, and scissors were read out by the computer in random order at 4s intervals, and opponents were instructed to spontaneously select one of these words. Next, the movies and corresponding read-out words were presented to a separate group of players who were instructed to observe and play against the recorded opponents. One out of ten naive players managed to detect pupil dilations and relate them to the timing of their opponent’s decisions, and thereby significantly increased their probability to win the game. Once instructed about the pupil’s usefulness, all players performed above chance and increased performance by ~50% on average. Since in 40% of trials the pupil contained invalid information about the opponent's choice, we tested a fresh set of 10 players against opponents for which invalid trials were removed. Six of these naïve players exploited the pupil and performed above chance, but most remained unaware of their strategy. This is the first demonstration that people implicitly exploit a subconscious visual skill to gain advantage in a competitive game scenario.